Wettability is an important phenomenon greatly impacting specifics of fluid distribution and propagation in a porous media. Thus, being a key parameter for characterizing oil formation and simulation, wettability greatly affects rock filtration properties, like relative permeability and displacement coefficient.
A wetting degree is characterized by a wetting angle. A wetting angle (or a contact wetting angle) is an angle made up by planes tangent to interface surfaces limiting a wetting fluid. A vertex of the angle lies on a boundary of three-phases.
A measurement of a contact wetting angle value is one of the most common methods of wettability measurement. Contact wetting angle is determined geometrically as an angle on a boundary of three-phases—a fluid, a gas, a solid body. The contact angle is very important for understanding material surface properties—adhesion, wettability and free energy of the system in general.
Two different approaches are commonly used to measure a contact wetting angle: an optical measurement and a force measurement of surface tension (tensiometry). Optical tensiometry includes observation of a sessile drop of a test liquid on a surface of a solid material. In force tensiometry, forces of interaction between a solid body and a test liquid are measured. Most known methods for a contact angle measurement are as follows: a sessile drop method, a capillary rise method, a tilted plate method, and an immersion plate method (cf., D. N. Rao, M. G. Girard, “A new technique for reservoir wettability characterization,” J. Can. Pet. Technol., 35, 31-39 (1996), or D. N. Rao, “Measurements of dynamic contact angles in solid-liquid-liquid systems at elevated pressures and temperatures,” Colloids Surf, 206, 203-216 (2002)).
However, these methods do not take into account roughness and heterogeneity of a surface and potential complex geology of a pore structure. On a smooth surface, a contact wetting angle is fixed, while on a sample surface with sharp edges, the contact wetting angles are different. Also, a contact angle is measured on a single mineral, whereas a core contains numerous minerals with impurities. Finally, organic compounds absorbed on a sample surface strongly effect wetting properties.
The known Amott method (E. Amott, “Observations Relating to the Wettability of Porous Media,” Trans, AIME, 216, 156-162, 1959) combined imbibitions and forced displacement to measure an average wettability of a rock sample. The Amott method is based on the fact that a wetting fluid can spontaneously imbibe into a core simultaneously displacing a non-wetting fluid. The ratio of a spontaneous imbibition to a forced imbibition is used to reduce the influence of other factors, such as viscosity and initial rock saturation. Other scientists use modifications of the Amott method: Amott-Harvey method and USBM (cf., J. C. Trantham, R. L. Clampitt, “Determination of Oil Saturation After Waterflooding in an Oil-Wet Reservoir—The North Burbank Unit, Tract 97 Project,” JPT, 491-500 (1977).
A disadvantage of the Amott method and its modifications is that the methods have a big error when applied to samples with neutral wettability or when applied to samples with small sizes (less than 1 inch).
A method for determining wettability based on calorimetric measurements has been actively developed. Calorimeters have been used for a long time to study interaction between fluids and surfaces. calorimetry helps to determine thermodynamic state functions, such as internal energy or enthalpy related to a wetting process (cf., R. Denoyel, I. Beurroies, B. Lefevre, “Thermodynamics of wetting: information brought by microcalorimetry,” J. of Petr. Sci. and Eng., 45, 203-2126 2004).
An advantage of calorimetric methods is the possibility of conducting experiments in which initial and final states of a system are well determined, which is not always possible if other methods, like standard methods for contact angle measurement, are applied.
The disclosed method is directed to the study of wettability of porous samples and to determining with high accuracy a contact wettability angle for pores having different diameters.